1. Field of the Invention
The present invention relates generally to the field of valves, and more specifically, to a leak-free rotary valve with a worm gear situated between the magnetic actuator and the valve body.
2. Description of the Related Art
Quarter-turn valves require substantial torque to operate effectively. This is especially true of plug valves and butterfly valves. Breakaway torques required to open and close these valves can be huge. A simple mechanical connection to a handwheel or drive motor without gear reduction would be impractical in most valve sizes. For example, it would be impossible for most people to be able to manually open and close a four-inch plug valve without any gear reduction. By the same token, the motor required to produce the same amount of torque in an automatic actuator without gear reduction would be larger than the valve itself. When adding a magnetic coupling to the mix, it is even more impractical to produce large torques without gear reduction. The coupling also would be larger than the valve itself and very expensive to produce.
There is no getting around the need to provide gear reduction in the actuators that open and close these valves. Gear reduction makes it possible to manually open and close all but the largest of valves. Gear reduction also makes it possible to design practical automatic and control actuators for all quarter-turn valves. Most importantly, gear reduction makes it possible to design a magnetic coupling for all quarter-turn valves that is practical in size as well as in cost.
There are three basic types of gearing that can provide the necessary reduction tor practical actuation of quarter-turn valves: spur, or helical, gears; planetary gears; and worm gears. Spur gears are the most efficient of the three, but they require more room that the other types of gears to provide a given gear ratio. They also require that the input shaft into the gearset be offset from the output shaft or the gearset, which makes mounting the actuator more complicated. Planetary gears are also more efficient than worm gears. Because there is no offset between the input shaft and the output shaft, they take up less room than spur gears and can be mounted directly over the valve stem.
When using gears in retrofit valve actuators, there are other factors to consider besides efficiency. The following advantages are provided by worm gear designs:
(1) Worm drives provide a built-in braking system; i.e., they will not move when force is applied to the drive system from the downstream (reduction) side. This is important, particularly with butterfly valves in a partial open position. High fluid velocities inside the valve can deflect the position of the valve unless held rigidly by the actuator. This is also true with ball valves, although the forces are not as severe. With planetary or spur gear designs, a separate braking system must be implemented into the drive system because the gears do not provide braking.
(2) Worm driven dominate the quarter-turn valve actuation industry. They are by far the most popular method of reducing speed and increasing torque to the valve. In other words, they are accepted by the industry.
(3) Worm drives contain less gearing for a given reduction ratio. For example, to provide a reduction ration of 20:1, a stack of at least three planetary gearsets would be required. By contrast, a single worm drive can provide the same ratio. This makes the worm drive much more economical to manufacture.
(4) The worm drive gearbox is more compact than either the spur gear or the planetary gear designs, especially when providing reduction ratios of 20:1 or greater. Because the gearset contains only two gears, the gearbox can be made much smaller in volume. This is important in high-pressure applications where the size of the gearbox determines the wall thickness required to hold a given pressure. A larger gearbox requires a thicker wall to hold the same pressure that a smaller gearbox can hold with a smaller wall thickness. Again, this leads to a reduction in manufacturing costs.
Unlike other gear trains, the direction of transmission in a worm gear (input shaft versus output shaft) is not reversible when using large reduction ratios, due to the greater friction involved between the worm and worm-wheel, or gear. One cannot turn the gear by applying torque to the output shaft. In this case, the worm gear is considered to be self-locking. Technically speaking, this occurs when the tangent of the lead angle of the worm is less than the coefficient of friction between the worm and the gear. High gear reduction worm drives require a very shallow lead angle, so in most cases the lead angle is indeed less than the coefficient of friction between the gear teeth; i.e., the worm gear is self-locking.
When actuating valves, the self-locking feature of the worm gear is especially advantageous. For example, butterfly valves have a tendency to move from a given position because the paddle of the valve is being pushed on by the fluid in the valve. This is especially prevalent when the valve is being used to throttle the now of fluid, i.e., when there is a large pressure drop as the fluid passes through the valve. In this case, ordinary gear trains will not be able to hold the paddle stationary. Instead, forces applied to the paddle by the fluid are converted to torque that will turn the output shaft of the gear train. Unless a braking mechanism is positioned somewhere between the drive motor (handwheel) and the valve, the valve will wander out of position, turning the motor or handwheel away from its intended position. With a high-reduction, self-locking worm gear, this does not happen because of the inherent self-locking feature of the gear train.